In recent years, the application range of, for example, magnetic recording/reproducing apparatuses, flexible disk apparatuses, and magnetic tape apparatuses, has increased remarkably, and the importance thereof has increased. Therefore, a technique has been developed for significantly improving the recording density of magnetic recording media used in these apparatuses. In particular, the introduction of an MR head and a PRML technique has significantly increased surface recording density. In recent years, with the development of GMR heads and TMR heads, the recording density of magnetic recording media has increased at a rate of about 100% per year. There is a demand for a further increase in the recording density of magnetic recording media. In order to meet such demand, it is necessary to improve the coercivity, signal-to-noise ratio (SNR), and resolution of the magnetic recording layer. In addition, in recent years, there has been an attempt to increase track density in addition to linear recording density, thereby improving surface recording density.
The latest magnetic recording/reproducing apparatus has a track density of 110 kTPI. However, when the track density increases, interference occurs between the magnetic recording information items of adjacent tracks, and a magnetization transition region, which is a boundary region therebetween, acts as a source of noise, which may cause a reduction in SNR. When the SNR is reduced, bit error rates deteriorate, which prevents improvements in recording density.
In order to improve the surface recording density, it is necessary to further reduce the size of each recording bit on the magnetic recording medium and maximize the magnetic film thickness and saturation magnetization for each recording bit. However, when the size of the recording bit is reduced, the minimum magnetization volume per bit is reduced, and recording data is erased due to magnetization reversal caused by heat fluctuation.
When the distance between the tracks is reduced, the magnetic recording/reproducing apparatus requires a very accurate track servo technique. In general, a method has been used which performs recording with a large track width and performs reproduction with a track width smaller than that during recording to minimize the influence of adjacent tracks. However, in this method, it is possible to minimize the influence between the tracks, but it is difficult to obtain a sufficient reproduction output. Therefore, it is difficult to ensure a sufficient SNR.
In order to solve the problem of heat fluctuation and ensure a sufficient SNR and a sufficient output, the following method has been proposed in which concave and convex portions are formed on the surface of a recording medium along the tracks to physically separate the recording tracks, thereby improving track density. Hereinafter, such a technique is referred to as a discrete track method, and a magnetic recording medium manufactured by the discrete track method is referred to as a discrete track medium. In addition, there has also been an attempt to manufacture so-called patterned media in which a data region in the same track is further divided.
As an example of discrete track media, a magnetic recording medium has been known in which a magnetic layer is formed on a non-magnetic substrate having an uneven pattern formed on the surface thereof so as to form magnetic recording tracks and servo signal patterns that are physically separated from each other (for example, Patent Document 1).
In the magnetic recording medium, a ferromagnetic layer is formed on the surface of a substrate on which a plurality of concave and convex portions is formed, with a soft magnetic layer interposed therebetween, and a protective film is formed on the surface of the ferromagnetic layer. In the magnetic recording medium, a magnetic recording region that is physically separated from its surroundings is formed in a convex region. According to the magnetic recording medium, it is possible to prevent the formation of domain walls in the soft magnetic layer. Therefore, it is possible to prevent an adverse effect by heat fluctuation and there is no interference between adjacent signals. As a result, it is possible to provide a high-density magnetic recording medium with less noise.
Examples of the discrete track method include a method of forming a magnetic recording layer including a number of thin films and physically forming tracks on the magnetic recording layer and a method of forming an uneven pattern on the surface of a substrate and forming a thin magnetic recording layer on the substrate (for example, Patent Documents 2 and 3).
In addition, a method has been proposed in which nitrogen or oxygen ions are implanted into a region between the magnetic tracks of a magnetic layer that has been formed in advance in the discrete track medium, or a laser beam is radiated onto the region to change the magnetic characteristics of the region (Patent Documents 4 to 6).
[Patent Document 1] JP-A-2004-164692
[Patent Document 2] JP-A-2004-178793
[Patent Document 3] JP-A-2004-178794
[Patent Document 4] JP-A-5-205257
[Patent Document 5] JP-A-2006-209952
[Patent Document 6] JP-A-2006-309841